1. Field of the Invention
The present invention relates to an oil pump driving device adapted to be mounted on a transmission for supplying working oil to lubricate the transmission and control gear shifts or speed changes in the transmission.
2. Description of the Prior Art
Transmissions for use on automobiles have many mechanisms or members therein that need to be lubricated, and it is necessary to supply such mechanisms or members with working oil for lubrication while the transmission is in operation. Hydraulically operated clutches and brakes that are used to effect gear shifts or speed changes in transmissions are also required to be supplied with working oil under a predetermined hydraulic pressure (line pressure) for controlling the clutches and brakes, i.e., the gear shifts or speed changes. Such working oil is supplied by an oil pump mounted on the transmission. Heretofore, the oil pump has been mechanically coupled to and driven by an engine with which the transmission is combined. For example, Japanese laid-open patent publication No. 55-60758 discloses a mechanism for driving an oil pump with an engine through a shaft that extends through the turbine shaft of a torque converter and is directly coupled to the output shaft of the engine.
Japanese patent publication No. 53-22214 discloses a transmission which is supplied with working oil by an oil pump that can be driven by an electric motor rather than an engine associated with the transmission.
Another transmission disclosed in Japanese patent publication No. 51-18082 is supplied with working oil by an oil pump that is driven by an engine during operation of the engine. When the engine is not in operation, the oil pump is driven by an electric motor.
The amount of working oil supplied to lubricate the transmission and the amount of working oil supply to control the transmission to effect gear shifts or speed changes are considered to be substantially constant regardless of the rotational speed of the engine. Actually, the amount of working oil supplied to lubricate the transmission slightly increases with the rotational speed of the engine. However, since the rate of increase of the amount of working oil supplied to lubricate the transmission is very small compared with an increase in the rotational speed of the engine, the amount of working oil supplied to lubricate the transmission may be regarded as being substantially constant.
As disclosed in Japanese laid-open patent publication No. 55-60756 and Japanese patent publication No. 51-18082, if the oil pump is driven by the engine, then the working oil is discharged by the oil pump at a rate that increases substantially in proportion to the rotational speed of the engine. When the engine rotates in a high speed range, only a portion of the working oil discharged by the oil pump is put to use, and hence a large energy loss results from the operation of the oil pump.
More specifically, the amount of working oil that is required by the transmission for its operation has no direct bearing on the rotational speed of the engine. The minimum requirement is that the transmission be supplied with a predetermined amount of working oil when the engine is idling, i.e., the engine is rotating at a lowest speed. Therefore, the oil pump that is used should have a displacement capacity such that it can supply the transmission with such a predetermined amount of working oil when the engine is at an idling speed. As the rotational speed of the engine increases, the oil pump discharges an excessive amount of oil in proportion to the rotational speed of the engine, and hence the energy loss of the oil pump also increases.
According to Japanese patent publication No. 51-18082, the oil pump can be driven by the electric motor as described above. The purpose of the disclosed system is to drive the oil pump with the electric motor when the engine is shut off for emission control at the time the automobile is stopped at an intersection or the like. Therefore, it is necessary for the oil pump to have such a displacement capacity as to discharge working oil in an amount that is necessary to lubricate the transmission and control gear shifts in the transmission when the engine is idling. As a result, as the rotational speed of the engine increases, the energy loss of the oil pump increases.
The oil pump disclosed in Japanese patent publication No. 53-22214 is free of the above problem as it is driven by the electric motor rather than the engine. However, the electric motor suffers a durability problem because it needs to be energized at all times insofar as the engine is in operation. In view of battery capacity limitations, the electric motor should preferably be as small in size as possible for minimum electric energy consumption, but its durability generally tends to decrease as the size is reduced. Larger electric motors are generally more durable, but consume more electric energy, and demand larger batteries and hence larger oil pump driving systems.